Lythrum carthagenense Jacquin, Enum. Syst. Pl., 22. 1760; Balsamona pinto Vandelli; Cuphea balsamona Chamisso & Schlechtendal; Parsonsia pinto (Vandelli) A. Heller

Herbs annual, [subshrubs], 1–6 dm, with fibrous roots. Stems erect to decumbent and spreading, usually much-branched, hispid and setose, sometimes also puberulent. Leaves opposite, subsessile or sessile; petiole 0–2 mm; blade broadly elliptic to lanceolate, 12–55 × 5–25 mm, base attenuate. Racemes leafy. Pedicels 1–2 mm. Flowers alternate, 1 interpet­iolar, with 1–3 flowers on axillary branchlets; floral tube purple adaxially and distally, or green throughout, 4–6 × 1–1.5 mm, glabrous except veins sparsely and coarsely setose; base rounded or a descending spur, 0.5 mm; inner surface glabrous; epicalyx segments thick, often terminated by a bristle; sepals equal; petals 6, deep purple or rose purple, subspatulate, subequal, 1.5–2.5 × 0.5–1 mm; stamens 11, extending 2/3 distance to sinus of sepals. Seeds (4–)6(–9), elliptic to suborbiculate in outline, 1.5–1.7 × 0.2–1.5 mm, mar­gin narrow, flat­tened, thin. 2n = 16.

Flowering late spring–fall. Atlantic and Gulf coastal plain, ditches, margins of moist woods, roadsides, moist open, disturbed areas; 0–200 m; introduced; Ala., Ark., Fla., Ga., La., Miss., N.C., S.C., Tenn., Tex.; Mexico; Central America; South America; introduced also in Pacific Islands (Fiji, Guam, Hawaii, Philippines), Australia.

The weedy, self-fertilizing Cuphea carthagenensis is the most widely distributed species of the genus and one of the more common in South America. It was first collected in the United States in Florida and North Carolina in the 1920s. Fossilized pollen very similar to pollen of C. carthagenensis and close relatives is known from the late Miocene of Alabama (S. A. Graham 2013). The species flowers year-round in subtropical and tropical regions.